Data and voice communications systems often employ frequency division multiplexing (FDM) to increase the bandwidth of the system. FDM allows two or more simultaneous and continuous channels to be derived from a shared access transmission medium. FDM assigns separate portions of the available frequency spectrum, separated by some minimal channel spacing within a block of spectrum, to each of the individual channels. However, FDM only provides for a fixed number of physical channels (i.e. separate frequencies) in a shared access network.
Therefore, many communication systems utilize time division multiplexing (TDM) to provide more channels and to increase the number of users that may operate on a shared access network. TDM provides time division multiple access (TDMA) in which users share a carrier frequency in the communications system by requesting and being granted a limited amount of time to transmit over the time division multiplex channel (time slots). In effect, each user gets assigned a different time slot on the same frequency. In this way many users can share the same frequency. The occurrence of simultaneous transmissions by two or more stations is called a collision. In data and voice networks using contention based request slots in a shared access media, such as for example cable, wireless and power data and voice networks, the signals often collide when attempting to randomly access the shared access network.
When a collision occurs on an Ethernet or other baseband network, the voltages of the received signals add in a linear fashion. Therefore, collision detection systems for baseband networks typically monitor the signal level on the coaxial cable or other transmission medium and a call a collision when the received signal level is in excess of that generated by the local transmitter. Further, because the voltages add linearly for baseband networks, the number of signals colliding may also be determined by dividing the peak signal level on the coaxial cable by the nominal amplitude of a single transmission.
However, in RF transmission systems (such as for example, a cable or wireless network) the information signal is modulated onto an RF carrier. Further, in typical RF transmission systems the phase of the signals transmitted by the individual stations are not synchronized. The lack of phase synchronization may create a random phase offset between the signals transmitted by the individual stations. Therefore, in a typical RF transmission system the phase of the signals that collide at the receiver may not be aligned and the signal level of the received signal may not directly correlate, in a linear manner, to the number of signals that collided. In fact, the random phase offset between the colliding signals may result in signals adding constructively (in phase), destructively (180 degrees out of phase) or at any level between the two extremes. Therefore, relatively ideal collision detection schemes employed in baseband networks may not provide accurate collision detection in a shared access RF network.
Frequently in typical request-grant systems, the station or modem assumes that a collision has occurred when a grant or grant pending indication for a particular request is not present in the next downstream media allocation and partitioning (MAP) cycle. Thus, in these typical systems, conflict resolution is delayed until the next available MAP cycle.
In current shared access RF networks such as for example, Data Over Cable Service Interface Specification (DOCSIS), the cable modem termination system (CMTS) may attempt to infer that a collision has occurred via physical layer or protocol algorithms. The CMTS may for example, call a collision event based on differences in signal to noise ratio between the preamble and the packet, or may infer that a collision event has occurred via protocol state transitions. Alternatively, a collision event may also be called in current systems when there is energy in a contention mini-slot but the data in that mini-slot can not be decoded due to a failed header check sequence (HCS) or failed synchronization. However, conventional collision detection systems may not accurately distinguish errors due to a collision of two or more signals from errors due to other channel impediments that may otherwise corrupt the channel.
Current systems typically incorporate contention resolution algorithms (CRAs) along with collision detection algorithms to minimize the delay in re-transmitting access requests. Contention resolution algorithms typically utilize information supplied by the CMTS to control the retransmission of failed requests as well as other transmission parameters. In this context, the information extracted from the result of the transmission is referred to as feedback.
In operation the CMTS of a DOCSIS compatible shared access RF network typically collects information on how many total collisions occur in the system. Typical DOCSIS compatible collision detection algorithms generate binary feedback in the form of collision (C)/non-collision (NC) decisions and do not estimate the number of users or signals involved in each collision event. However, theoretical studies have shown CRAs may more readily distinguish collisions from errors if the CMTS or central controller not only detects the occurrence of a collision but also estimates the number of signals that collided (also referred to as multiplicity of collision).
However, due to the non-linear nature of collision events in shared access RF networks it is difficult to directly obtain accurate feedback on the multiplicity of collision. Therefore, current multiplicity estimators are often protocol-based and estimate the multiplicity of collision in accordance with the number of collisions that previously occurred on the system. Protocol methods directly depend on the CRA algorithm used because future collisions depend on how often the users retransmit a failed transmission.
Therefore, it would be advantageous to provide a physical layer detector for estimating the multiplicity of collision based on channel information.